The radiofrequency signal receiver of the electronic device can be used for example in a satellite navigation system of the GLONASS or GALILEO or GPS type.
The radiofrequency signal receiver of the electronic device can be of the GPS type, wherein the radiofrequency signal reception and shaping means generate intermediate signals, which are frequency converted for a correlation stage. This correlation stage is composed of several correlation channels for receiving the intermediate signals in order to correlate them with carrier frequency and specific code replicas from satellites to be sought and tracked. Preferably, the receiver can include a larger number of correlation channels than the number of visible satellites for the electronic device worn by a user situated on the Earth.
In the case of a GPS system, any radiofrequency signal receiver can receive GPS signals from satellites in orbit. The ground distance between the receiver and a visible satellite can be between 20,000 km, when one of the satellites is at the zenith, and 26,000 km, when one of the satellites is on a tangent point to the Earth's surface, i.e. on the horizon.
Currently, 30 satellites are placed in orbit at a distance close to 20,200 km above the surface of the Earth on 6 orbital planes each inclined by 55° in relation to the equator. The time taken by a satellite to complete one rotation in obit in order to return to the same point above the Earth is around 12 hours. The distribution of the satellites in orbit allows a terrestrial GPS receiver to receive GPS signals for example from at least four visible satellites in order to determine its position, speed and the local time for example. The GPS message from the GPS signals contains ephemeredes and almanac data used in particular for calculating the X, Y and Z position, speed and local time.
All of this known ephemeredes and almanac data is stored in the storage means of the portable electronic device. Thus, using an approximate known position of the receiver, it is possible to configure said receiver to search and track satellites that should be visible at the moment of search.
In the case of a navigation system, the use of a compass in addition to the radiofrequency signal receiver is known in order to provide the user of the portable device with information regarding the target to be reached.
In this regard, U.S. Pat. No. 5,790,477 can be cited, which discloses a watch provided with a GPS receiver, and a magnetic compass. In this watch, the coordinate points of a geographical target to be reached have to be entered and stored. In order to facilitate the operations for calculating the direction to be followed, the compass provides data relating mainly to geographical north. After processing the GPS signals and the data from the compass, a display device of the watch indicates the direction to be followed to reach the desired target.
In this case the compass is used only for the watch to give information to the user in relation to the direction to take in order to get to the geographical place to reach.
International Patent Application No. WO 92/21990 discloses a portable electronic device as a navigation system direction indicator. In order to do this, the device includes a GPS receiver, and a magnetic compass. The GPS receiver and compass generate signals that are processed by a processing unit in order to provided a direction signal. A display screen of the portable device displays an arrow indicating the direction to be followed in order to reach a selected geographical target.
The magnetic compass of this portable electronic device has no functions other than providing information to the processing unit to enable it to display the direction of the geographical place to be reached.
Since the electronic device of the present invention is worn on a user's wrist, the user's body can form an obstacle to reception of radiofrequency signals from certain visible satellites when the device is in the reading position. This can constitute a drawback if many of the visible satellites to be sought are hidden by the user's body.
It is thus a main object of the invention to provide a portable electronic device provided with a radiofrequency receiver and a compass to assist said receiver in order to overcome the aforementioned drawbacks.